The invention relates to a method and device for monitoring, during operation, the performance of an electrical spark gap, in particular a spark plug of a combustion engine, which is connected to a device for generating electrical high voltage, the current arising during the flashover of the spark gap being detected, converted into a measuring signal representative of the instantaneous operational state of the spark gap, and analyzed.
Electrical spark gaps are in practice most commonly used for igniting or initiating the ignition of a combustion process, for example the ignition of gas-fuelled boilers or heating stoves or the combustion process of the so-called spark-ignition engines (petrol and gas engines according to the Otto principle). Spark gaps for use in combustion engines are generally known under the name spark plug.
Each spark causes wear of the electrodes, which increases the distance or gap between the electrodes. Because the electrical high voltage generated is limited in amplitude in practical systems, flashover between the electrodes will no longer occur beyond a certain spacing or gap size. Adjustment of the electrode spacing or replacement of the spark gap or spark plug, respectively, is then necessary. For the sake of an undisturbed and efficient combustion process it is also necessary to keep the gap size within certain limits.
Until now, establishing the time when a spark plug has to be replaced or adjusted has, in practice been done either periodically, possibly in combination with an inspection, or on the basis of detecting a failure, for example cylinder failure in a combustion engine. A periodical inspection is time-consuming and consequently expensive. Because of electrode fouling, it is, moreover, difficult to establish optically to what extent a spark plug is worn. A very serious drawback is the fact that, for the purpose of said inspection, the process controlled by the spark gap or spark plug has to be stopped and the spark plugs have to be removed. This applies particularly to combustion engines working continuously or stationarily, for example for the purpose of driving electrical generators, in which case stopping the process controlled by the engines, that is to say the interruption of the generation of energy, can be economically very disadvantageous.
U.S. Pat. No. 4,558,280 discloses a method and device for monitoring during operation, the spark plugs of a combustion engine, in which method and device the spark plug current arising during the flashover is detected. The measured current is converted into a measuring signal representative of the instantaneous operational state of the spark plug by means of an electronic processing circuit. For the purpose of determining the performance of a spark plug, a comparative measurement is performed, the measuring signal from the spark plug in question being compared with the instantaneous average of the measuring signals of all the spark plugs.
This measurement method however only makes it possible to detect whether the spark plug in question deviates, with regard to performance, from the average of all the other spark plugs. Checking the instantaneous operational state of the spark plugs relative to a calibration value is not possible by this method. In essence, only fault detection can be carried out using the method and circuit according to this US patent. Aging of the spark plugs, a change in the electrode spacing relative to the spacing at installation, and the like, cannot be detected, nor is the device suitable for monitoring one single spark gap, for example in a boiler or the like.
U.S. Pat. No. 4,825,167 describes a device and method for the testing of spark plugs under simulated operational conditions. In this case, too, the current in the spark plug during flashover therein is detected, and the measuring signal subsequently being compared to predetermined reference data, and any fault signal is thereby produced.
In addition to the disadvantage of testing under simulated operating conditions, for which purpose the process controlled by the spark plug has to be interrupted, this method has the practical drawback that it requires the previous introduction of reference data. These reference data may, however, vary from engine to engine and as a function of the type of the spark plug used. Extensive management of reference data is therefore required, and the possibility of mistakes in introducing these reference data, and as a consequence, the taking of wrong decisions is by no means inconceivable.